1. Field of the Invention
This invention relates to a focusing device for a television electron microscope and in particular to an improved focusing device for an electron microscope permitting observation of an enlarged image through a television.
2. Description of the Related Arts
Heretofore, a device usually called a wobbler has been used as a focusing device for a general transmission type electron microscope. A focusing device for a transmission type electron microscope using such a wobbler has been disclosed in Japanese Patent Application, Un-examined Publication No. 78/15738 (Post-examination Publication No. 83/24899), filed on July 28, 1976 in Japan by the same applicant as this application and laid open on Feb. 14, 1978.
This wobbler is a device varying in time the incident angle of an electron beam projected on a sample with respect to it. It utilizes the fact that, when the focal point of the electron microscope is not well adjusted, the image of the sample enlarged on the image formation plane is seen fluctuating, as the incident angle varies, and when the focal point is well adjusted, the position of the image doesn't change and it is seen standing still, even if the incident angle varies. Consequently, the user observes movement of the image of the sample on a fluorescent plate, while operating the wobbler and adjusts the focal point by varying current flowing in the objective lens of the transmission type electron microscope and thus regulating the focal length so that the movement of the image can be no more observed. This method is widely utilized as a simple focusing method. The application of this device to a television electron microscope has given rise to a problematical point that, when the incident angle of the electron beam varies during the horizontal scanning of the television, if the focal point is not well adjusted, fluctuations of the image on the display of the television are observed in the form of a fuzziness of the image and therefore it is difficult to adjust the focal point with a high precision.